Books don’t emit light in the visible spectrum so it does not make it easier on the eyes in low light conditions.
Historically it’s also cheaper because paper is some shade of white and it’s cheaper to not soak the page in ink.
It’s way more expensive.
Same reason that it’s uncommon for any page to have most of the page covered in ink, regardless of whether it’s a book or a sheaf of papers or whatever. Ink costs something, and it’s cheaper to put ink on a little bit of the page than it is to put ink on everything but a little bit of the page. Unless there’s a compelling reason to do otherwise, you take the cheaper route.
What if you use white ink on black paper?
You need to dye the paper. And the white ink would need to be suuuper opaque to even have a chance to be readable
Which is basically impossible to do
